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LE MANS: The Aerodynamic Influence of Louvers on Prototypes
Written by: Marshall Pruett   
06/14/2008
Le Mans, France
 
Manufacturers like Pescarolo opt for large, full-length 'alligator teeth' louvers on their LMP1 cars, despite their impact on straighline speed. (Photo: Marshall Pruett) » More Photos


Have you ever wondered what purpose the jagged appendages mounted over the front wheels of a prototype happen to serve? Those appendages are called ‘louvers,’ and despite their odd and varying shapes, these highly refined pieces of aerodynamic kit play a highly influential role in the performance of a sportscar.

Louvers are used over the fenders of sportscars to relive high pressure that builds up from the fast spinning tire, and from the complex interaction of air flowing in, out, and through the wheel arch.

The general rule is the more air you can extract from the wheel well, the better. On street courses, prototype teams will usually sport the biggest, longest, and most aggressive forms of louvers to vent air and create downforce. Another aspect of most louvers is their taller height (to give air a ‘chimney’ to flow out from) – that extra height adds more volume to the outer profile of the car—making a bigger hole for the passing air to go over and thus reducing straightline speed.
Contrasting Pescarolo, Audi have gone to a flush upper surface on their bodywork, using recessed louvers at the only the rear. (Photo: Marshall Pruett) » More Photos

Yet at Le Mans, a track where excessive downforce will reduce the quest for top speeds on the many long straights, most teams go small, opting for minimal venting.

Placement of louvers is also critical—based upon the aerodynamic layout of a prototype’s fenders, louvers can run the full length of the fender, only half the length (either the leading edge of the fender or the trailing edge), or as is often seen at Le Mans, blanked off altogether. If there’s a better compromise on reducing some of the high pressure air without greatly impacting straighline speed, it’s in the use of louvers that blank off the first half of the fender, only to sprout a series of louvers towards the back half of the fender.

But not everybody has gotten with the program. Another general rule—one that works brilliantly for small teams that lack the aero testing budgets that an Audi or Peugeot might have, is to mimic what the big teams do at a track like Le Mans.
Porsche has gone a slightly different route, still retaining a recessed design, but employing two small louvers at the front and back of the wheel arch. (Photo: Marshall Pruett) » More Photos

Peugeot and Audi have both gone in the correct direction by going to ‘half-rearward’ louvers. Porsche, with their LMP2 RS Spyder, have come up with a novel approach, using two recessed (flush to the air passing over the top of the car, unlike Peugeot or Audi’s that stick up into the air) louvers at the leading edge, and two near the trailing edge. The middle is mostly blanked off.

Rollcentre Racing’s Pescarolo sports full non-Le Mans louvers—very tall, aggressive, and that run the full length of the fender. The same goes for the Terramos Courage LMP2 team.

The factory Courage-Oreca squad has come up with a much tidier arrangement, with a recessed ‘half-rearward’ solution.

While the many differences in louvers are easily spotted, their effectiveness in tuning a car for speed and downforce is more of a black art. The differences between full louvers like Rollcentre’s and fully blanked off can mean a difference of 6MPH in straighline speed.

To get a better appreciation of the influence louvers play in sportscar racing, our friend Brad Kettler from Audi shared some fascinating tales of their impact on performance.
Peugeot has gone for a 'half-rearward' arrangement, with raised louvers, but tucked back enough so they don't meet the air directly. It's interesting to note their rivals at Audi prefer their louvers in the same place, albeit recessed. (Photo: Marshall Pr » More Photos

“I really think a lot of it the louver design is predicated, at least on the cars that I know about, on the exit air from the brakes -- how the exit air from the brakes is dealt with. And it’s also trying to get some of the lift element out of underneath the wheel housings. Going back a couple of years here, I know the Pescarolo’s have always used a lot of louvers at Le Mans; I don’t know why and I can’t put my finger on it and it doesn’t
make a lot of sense to me, but obviously I don’t work for them so they must have a reason, but not one I know that works…”

Over the years of developing the Audi R10’s predecessor, a wide variety of louver packages were developed and refined to suit each circuit. “In general terms, what we found in the past with the R8’s and so forth is that we did have varying louver styles based on the type of track, and you want to get as much of the louver as possible out of the frontal area of the car so that the direct impact of the aero on the face of the car is not affected by the louvers so much, and use it as an aid to get the air out from underneath the bodywork.

“And I know that after that, louver design was really predicated on how the wheel ventilated and how the thermal air from the brakes ventilated and how much you had of it in what part of the bodywork. I mean, I think a lot of it, if we looked into it a little more, if we looked behind the louver at the side pod you would start to find some trend there that matches up at either one. I’m sure that in the case of the Porsche [RS Spyder], with small louvers at the front and louvers at the back, that’s really based on just being designed using tufted air or a manometer being put in the wheel house capture data on how best to get the [high] pressure out. There’s no real reason to use them.”
Courage-Oreca, like Audi, opt for recessed and half-rearward. (Photo: Marshall Pruett) » More Photos

Although most prototypes look similar and, each model has different design concepts for inner and outer airflow – each manufacturer opts for a slightly different louver package that suits their car, and their car alone. “I’m sure that’s where it comes from, and I’m sure all of the characteristics of the different cars are, you know, the high pressure is under the bodywork and I’m sure what goes into doing that. Like I said, I know from the R8 for sure, and from this car as well that you want to get them off of the frontal area of the car because they are very draggy, draggy elements and so forth. That’s what we learned worked with that particular car. And of course these [on the R10 at Le Mans] don’t; the ones that we use in the United States -- the ‘draggy-ness’ does offer a bit of downforce with it and these don’t. This is just drag. They must be incredibly efficient aerodynamically somewhere else if they’ve got those big louvers on. Otherwise, they’d be faster without them.”

What’s baffling is that in a paddock filled with major manufacturers like Peugeot and Audi spending a fortune to lead prototype aero developments, a smaller company like Radical or Pescarolo wouldn’t observe the major difference in louver packages, and consider adopting something that’s been proven as an outright advantage. In this case, would it be feasible for them to copy and apply what the competition is doing?
The Terramos Courage--a private entry, uses large, full-length louvers, where the factory Courage-Oreca team has developed a better and faster solution. (Photo: Marshall Pruett) » More Photos

“Oh that’s very viable, I mean a lot of this business is based on that kind of stuff. We all look at each other’s cars with a lot of interest to see how that stuff is applied and so forth but in the case of like the Radical for example, I know that in the original version of that car, the original Peter Elleray version, didn’t have wind tunnel development, it was all CFD, all drawn on a computer. Later on it was put in a wind tunnel, I think my facts are right, and it was proven to be pretty good.”

That’s a great point; smaller cars like the Radical have indeed proven to be good, but to become great, perfecting smaller areas of development such as louver design and implementation makes all the difference between achieving wins or settling for mediocrity.
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